1. Field of the Invention
The invention relates to the use of nanofibers as membranes for controlling molecular transport. These devices provide nanoscale control of molecular transport by mimicking biological cellular membranes. Semi-permeable membranes can be created from the directed self-assembly of nanofibers, allowing for the passage of molecules smaller than the wall-to-wall spacing of the nanofibers. The diffusion limits can be controlled by the separation of the fibers, both laterally and along the direction of transport. Chemical potential gradients can be engineered and used to direct transport. These membranes can involve chemical derivatization of the fibers to further affect the diffusion limits or affect selective permeability or facilitated transport. Additionally, individually addressable nanofiber electrodes can be integrated with the membrane to provide an electrical driving force for transport and an electronic interface to the fluid for control and detection.
2. Description of the Related Art
Much of current nanotechnology is deriving its inspiration from natural systems. The approach of integrating nanoscale functionalities (i.e., sensing, signal processing and actuation) can be considered within the context of a biological cell. A cell interacts with its environment by “sensing” chemical signatures. This chemical information is processed by affecting the chemical networks within the cell that may result in some sort of actuation, such as production of another chemical. Biological cells are typically a few microns in diameter (bacterial cells) with membranes on the order of a few nanometers thick—dimensions that are within the range of typical nanomaterials and micromachining techniques. These dimensional characteristics of cells are fairly well conserved and are undoubtedly critical for cell function. For example, short distances (nm-μm) enable intra- and inter-cellular communication by a simple transport mechanism such as diffusion. Even large proteins can diffuse across distances of a few microns in only a few tenths of a second. Also, the small fluid volume of a cell allows for only a few thousand molecules to result in a significant concentration difference. Reducing the scale of artificial systems to these dimensions will be critical for their effective implementation as well as for interfacing to biological systems. To mimic the properties of biological cells, details at the nanometer scale must be combined with structures that are microns or millimeters in extent. Therefore, a significant challenge and opportunity exists in the construction of components with engineered features on multiple length scales that scale six orders of magnitude (nm. to mm.).
Cells possess many features that are worthy of emulation. Amazingly, cell function can be extremely diverse yet utilize a common set of building blocks. Cells can operate under a wide range of environmental conditions with efficiencies unmatched by artificial creations. Further, cells can be highly specialized, carry out tens of thousands of chemical reactions in parallel, and communicate with other cells.
Artificial creations can have greatly increased functionality with the synthesis of even simple cell-like structures. The use of a cellular structure is a universal feature throughout nature. The variations and functions of naturally occurring cells indicate that applications of cellular mimics may be limitless. However, just as creating structures on the scale of natural systems presents great opportunities, it also presents significant challenges. The most significant challenge in mimicking cell structures is fabricating appropriate membrane structures that contain molecular-scale pores. A fluid, lipid bilayer membrane envelopes natural cells. It serves as both a container and a controller of the chemical reactions inside the cell. Reagents are exchanged with the neighboring environment through the creation of chemical potential gradients, or actively transported using enzymatic systems. This sort of molecular transport is distinct from bulk fluidic-based transport. Membrane transport is molecule specific and is accomplished either passively, based on chemical potential gradients, or actively using energy transduction schemes. For mimicking cellular structures, the incorporation of semi-permeable barriers, or membranes, is a necessity. These membranes must be able to selectively control the transport of molecular species, requiring engineering on the nanometer scale.
There are many material approaches to mimicking membranes (Fendler, “Polymerized Surfactant Vesicles; Novel Membrane Mimetic Systems”, Science, 223, 888-894, 1994). One approach to the construction of synthetic membranes involves the classical techniques of forming lipid vesicles. Mechanical agitation, or sonication, of phospholipids forms discrete vesicles, or liposomes. Such liposomes are typically small, however other techniques can produce liposomes with diameters on the order of 100 microns (Oku et al., “Preparation of giant liposomes,” Biochimica et Biophysica Acta, 692, 384-388, 1982). Planar supported bilayers can also be constructed from phospholipids (McConnell et al., “Supported planar membranes in studies of cell-cell recognition in the immune system”, Biochimica et Biophysica Acta, 864, 95-106, 1986). Micropatterning of these planar supported bilayers has been accomplished using lithographically patterned grids (Groves et al., “Micropatterning fluid lipid bilayers on solid supports,” Science, 275, 651-653, 1997; and Cremer et al., “Writing and erasing barriers to lateral mobility into fluid phospholipid bilayers,” Langmuir, 15, 3893-3896, 1999). This method can confine membrane components to specific grid elements but does not result in fluid filled cells of practical utility as the planar supported membranes rest on only a 10-20 Å thick aqueous layer. These artificial membranes, composed of naturally occurring membrane components, have been useful in understanding the physical and biological properties of cell membranes (e.g. permeability, molecular events in signal transduction). The use of liposomes, or polymeric vesicles, has also been considered for the creation of synthetic cells such as those suitable for targeted drug delivery (Fendler, “Polymerized Surfactant Vesicles; Novel Membrane Mimetic Systems”, Science, 223, 888-894, 1994; and Hammer and Discher, “Synthetic Cells—Self-Assembling Polymer Membranes and Bioadhesive Colloids”, Annu. Rev. Mater. Res., 31, 387-404, 2001). The construction of vesicles with integrated functionalities may be possible with such systems. However, the design of discrete, joined cells with specific pore structures is not obvious with this approach. The seemingly beneficial structure that is both fluid and self-assembling also allows for free diffusion and reshaping of the membrane. Additionally, the long-term stability of such structures may preclude their use in practical applications.
Other membrane structures have been constructed from rigid polymeric films or metals containing nanopores (Martin, “Nanomaterials: A membrane-based synthetic approach,” Science, 266, 1961-1966, 1994). Polyester, polycarbonate, or aluminum can be etched to create pore diameters as small as a few nanometers. Extremely small pores can also be created in glass by the repeated drawing and bundling of glass capillaries containing an etchable core. Tonucci and co-workers have prepared nanochannel glass templates useful for creating porous membranes of various metals (Pearson and Tonucci, “Nanochannel glass replica membranes,” Science, 270, 68-70, 1995). The pore diameters of these structures can be as small as a few tens of nanometers. Silicon substrates can also be used as nanoporous substrates. For example, block copolymer lithography has been used to prepare pores on the order of 20 nanometers at a pitch of 40 nanometers in silicon nitride-coated silicon wafers (Park et al., “Block copolymer lithography: Periodic arrays of ˜1011 holes in 1 square centimeter,” Science, 276, 1401-1404, 1997). Nanopores have also been created in silicon by selective etching of carefully engineered oxide layers. Pores as small as 18 nanometers have been prepared for the construction of a silicon “biocapsule” useful for immunoisolation (Desai et al., “Microfabricated immunoisolating biocapsules,” Biotechnology and Bioengineering, 57 (1), 118-120, 1998). However, as with the lipid bilayers discussed above, these structures are planar in format and do not lend themselves to the design of discrete, fluidly joined cells. Additionally, membranes based on these materials are typically thick (˜100 μm) relative to biological membranes. This can considerably limit the rate at which material can transfer across a membrane potentially limiting chemical transfer rates compared to natural cell membranes.
An alternate approach to creating sieving structures is to create obstacles that are perpendicular to the direction of transport. For example, micromachined posts have been used as synthetic gel media in the electrophoretic separation of biomolecules (Volkmuth and Austin, “DNA electrophoresis in microlithographic arrays,” Nature, 358, 600-602, 1992). These posts have been constructed, using electron beam lithography, with features as small as 100 nm and with a monolithic fluid enclosure (Turner et al., “Monolithic nanofluid sieving structures for DNA manipulation,” J. Vac. Sci. Technol. B, 16(6), 3835-3840, 1998). In this approach to molecular sieving, the distance between the outer edges of the obstacles creates the “pore”. The planned construction of these structures enables explicit definition of the separation capabilities, promising to be a superior alternative to the randomly arranged pores of polymer gels. In general, the limitations of conventional micromachining techniques prevent constructing such structures with molecular dimensions. Silicon etching techniques have been extended to the construction of nanopillars, however the aspect ratio of such pillars places great restriction on the height (Lewis and Ahmed, “Silicon nanopillars formed with gold colloidal particle masking,” J. Vac. Sci. Technol. B., 16(6), 2938-2941, 1998).
The arrangement of carbon nanotubes (Iijima, “Helical Microtubules of Graphitic Carbon,” Nature, 354(6348) 56-58, 1991) may provide an alternative approach to creating membrane structures. However, the self-assembly of these products into higher order structures will require improved technical advances. Template-based methods have been described that allow the ordering of nanoscale objects (Martin, “Nanomaterials: A membrane-based synthetic approach,” Science, 266, 1961-1966, 1994). For example, arrays of nanowires and nanorods have been described (Cao et al., “Well-aligned boron nanowire arrays”, 13(22), 1701-1704, 2001; Zhang et al., “Synthesis of ordered single crystal silicon nanowire arrays,” Adv. Mater., 13(16), 1238-1241, 2001; and Huang et al., “Room-temperature ultraviolet nanowire nanolasers,” Science, 292, 1897-1899, 2001). However, these techniques only allow for a limited control of nanowire position and morphology on a larger scale.
It has been recently demonstrated by the inventors of the present application that catalytically controlled growth provides a powerful method for directed self-assembly of vertically aligned carbon nanofibers into microscale and larger structures. (See Merkulov et al., “Shaping carbon nanostructures by controlling the synthesis process”, Appl. Phys. Lett., 79(8), 1178-1180, 2001; Merkulov et al., “Sharpening of carbon nanocone tips during plasma-enhanced chemical vapor growth”, Chem. Phys. Letts., 350(5-6), 381-385, 2001; Merkulov et al., “Alignment mechanism of carbon nanofibers produced by plasma-enhanced chemical-vapor deposition,” Appl. Phys. Lett., 79(18), 2970-2972, 2001; Merkulov et al., “Patterned growth of individual and multiple vertically aligned carbon nanofibers,” Appl. Phys. Lett., 76, 3555-3557, 2000; Merkulov et al., “Shaping carbon nanostructures by controlling the synthesis process”, Appl. Phys. Left., 79(8), 1178-1180, 2000; Guillorn et al., “Individually addressable vertically aligned carbon nanofiber-based electrochemical probes, J. Appl. Phys., 91(6), 3824-3828, 2002; Guillorn et al., “Operation of a gated field emitter using an individual carbon nanofiber cathode,” Appl. Phys. Lett., 79(21), 3506-3508, 2001; Guillorn et al., “Microfabricated field emission devices using carbon nanofibers as cathode elements,” J. Vac. Sci. Tech. B., 19(6), 2598-2601, 2001; and Guillorn et al., “Fabrication of Gated Cathode Structures Using an In-Situ Grown Vertically Aligned Carbon Nanofiber as a Field Emission Element,” J. Vac. Sci. Tech. B, 19, 573, 2001.) This “bottom-up” approach to construction allows control over the physical features of vertically aligned carbon nanofibers, and in combination with some “top-down” fabrication techniques (e.g. e-beam lithography) provides a powerful tool for the realization of complex microscale devices with functional nanoscale features. The ability to create fibers perpendicular to the substrate surface, with dimensions on the nanometer scale, provides the controlled synthesis and directed assembly required to realize membrane structures capable of controlling molecular transport. (See Zhang et al., “Controlled Particle Transport Across Vertically Aligned Carbon Nanofiber Barriers,” Applied Physics Letter, vol. 81, No. 1, 2002.)